The present invention relates to a polymer electrolyte fuel cell that works at ordinary temperature and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
The polymer electrolyte fuel cell causes a gaseous fuel, such as gaseous hydrogen, and an oxidant gas, such as the air, to be subjected to electrochemical reactions at gas diffusion electrodes, thereby generating the electricity and the heat simultaneously. A pair of catalytic reaction layers, which are mainly composed of carbon powder with a platinum metal catalyst carried thereon, are closely attached to opposite faces of a polymer electrolyte membrane, which selectively transports hydrogen ions. A pair of diffusion layers having both the gas permeability and the electric conductivity are further arranged on the respective outer faces of the catalytic reaction layers. The catalytic reaction layer and the diffusion layer constitute each electrode.
A pair of conductive separator plates are arranged across the membrane electrode assembly so as to mechanically fix the assembly and cause the assembly to electrically connect with the assembly in series. A specific part of the separator plate that is in contact with the electrode of the assembly has a gas flow path, which feeds a supply of reaction gas to the electrode and flows out the gas evolved by the reaction and the remaining excess gas.
The structure of each unit cell included in such a fuel cell is described below with the drawings.
FIG. 11 shows a unit cell in which a pair of electrodes 1, each having a catalytic layer 2, are arranged across a polymer electrolyte membrane 3 to yield a membrane electrode assembly, the circumferential part of the polymer electrolyte membrane 3 is interposed between a pair of sealing members 17, and a pair of separator plates 4 are arranged across the membrane electrode assembly. The separator plate 4 has a gas flow path 5 for feeding a supply of the gaseous fuel or a supply of the oxidant gas to the electrode 1. The sealing member 17 prevents the gaseous hydrogen as the gaseous fuel and the air as the oxidant gas from leaking out of the fuel cell or from being mixed with each other. A separator plate having the gas flow path formed on its one surface and a flow path of cooling water formed on its other surface is applied for every two unit cells. An 0 ring is interposed between the separator plates having the flow path of cooling water, in order to prevent a leak of the cooking water.
FIG. 12 shows another sealing technique for preventing leaks of the gases and the cooling water. This technique arranges gaskets 19, which are composed of an appropriate resin or metal and have a substantially identical thickness with that of the electrode 1, around the electrodes 1. In this structure, the clearance between a separate plate 4 and the gasket 19 is sealed with a grease 20 or an adhesive. The clearance between the separator plates having the flow path of the cooling water is also sealed with the grease or the adhesive.
FIG. 13 shows another example, in which membrane electrode assemblies (hereinafter referred to as MEAs), each of which is obtained by interposing a polymer electrolyte membrane between a pair of electrodes having an identical size with that of the polymer electrolyte membrane, and separator plates are alternately laid one upon another. This technique causes specific parts of the MEA that require the gas sealing property, to be previously impregnated with a resin 21, which has sealing effect and subsequently solidifies. The solidified resin ensures the gas sealing property between the MEA and the separator plate.
Most of the fuel cells have a laminate structure in which a large number of unit cells having the above configuration are laid one upon another. In the course of operation of the fuel cells, heat is produced with generation of the electric power. In the stack of unit cells, a cooling plate is provided for every one or two unit cells, in order to keep the cell temperature at a substantially fixed level and simultaneously enable the generated thermal energy to be unitized, for example, in the form of warm water. The cooling plate is generally a thin metal plate which a heat transfer medium, such as cooling water, flows through. Another possible application forms a flow path of cooling water on the rear face of the separator plate included in the unit cell, so as to make the separator plate function as the cooling plate as discussed above. In this case, a cooling water flow path is formed on the rear face of the separator plate, which is included in each unit cell, to make a flow of cooling water. In this structure, O rings and gaskets are required to seal the heat transfer medium, such as cooling water. The 0 rings in the seal should be compressed to ensure the sufficient electric conductivity across the cooling plate.
The stack of unit cells generally has a so-called internal manifold arrangement having gas inlets, gas outlets, and inlets and outlets of cooling water to and from the respective unit cells, which are generally called manifolds, inside the stack of unit cells. In the case where the reformed city gas is used as the gaseous fuel to drive the cells, however, the CO concentration rises in the downstream area of the flow path of the gaseous fuel. This may cause the electrode to be poisoned with CO, which results in lowering the temperature and thereby further accelerating the poisoning of the electrode. In order to relieve the deterioration of the cell performance, the external manifold type is noted as the structure that increases the length of the gas supply and exhaust unit between the manifold and each unit cell.
In either of the internal manifold type and the external manifold type, the required process lays a plurality of unit cells including the cooling units one upon another in one direction to provide a stack of unit cells, arranges a pair of end plates outside the stack of unit cells, and fixes the stack of unit cells between the pair of end plates with tie rods. It is naturally desirable to urge the whole face of each unit cell as uniformly as possible. In other words, it is desirable that the substantially uniform compressive force is applied to the whole laminating faces of the stack of unit cells. By taking into account the mechanical strength, the end plates and the tie rods are generally made of a metal material, such as stainless steel. These end plates and tie rods are electrically insulated from the stack of unit cells by insulator plates, so that the electric current does not run outside through the end plates. One improved technique of fastening makes the tie rods pierce the through holes formed in the separator plates. Another improved technique binds the whole stack of unit cells via the end plates with metal belts.
In any of the sealing methods shown in FIGS. 11 through 13, the constant compressive force is required to maintain the sufficient sealing property. One adopted structure inserts a coiled spring or a disc spring between the tie rod and the end plate. The compressive force ensures the electric contact between the respective constituents of the cells including the separator plates, the electrodes, and the electrolyte membranes.
In the structure that disposes the sealing members or O rings around the electrodes for the purpose of the seal of the gas, for example, the gaseous hydrogen or the air, a relatively large plane pressure is required. The adopted arrangement accordingly presses the sealing member or the sealing part between the pair of separator plates, so as to maintain the sufficient sealing effect. It is thus required to apply a relatively large compressive force constantly. This, however, makes the fastening mechanism including the end plates and the tie rods bulky and heavy in weight, while the fuel cell is required to have less total weight.
The long-term application of a pressure to the seals and the electrodes causes distortion of the constituents and thereby lowers the plane pressure required for the seals and the electrodes. A mechanism for absorbing the distortion is thus required in the fastening mechanism. One adopted mechanism for that purpose installs a spring on the end of the tie rod. This is another factor of making the whole fuel cell undesirably bulky.